The question is not so easy. A CAN bus can work
at any speed up to 1Mbps. Not all systems based on that bus operate at 250 Kbps. There are also devices in many areas of the industry that work
with 11-bit IDs instead of the 29 bits of NMEA2000.
I do not know these two details in the case of Dometic, but I can assure you that, although they are the same as in N2k (already difficult), you will not be able to share the bus because the network management will be different.
The J1939 system uses a network protocol identical to NMEA2000, and in spite of everything, note that the Maretron J2K100 has 2 CAN connectors. That has 2 possible reasons, one is the galvanic isolation, and another to avoid that possible PGN's own J1939 that do not interest anyone, are present in NMEA2000. (Filtering function). It also performs a J1939 compilation of the normalized data in NMEA2000 and generates the corresponding PGNs.
One more thing: J1938 is designed to use a group of instruments
for each engine
. When two motors with J1939 output are installed, it is necessary to assign an instance to each motor
when transferring the data to the common NMEA2000 network. This is done by configuring the J2K100. That is why it is necessary to install two Maretron when there are two motors.